The subject matter of the invention is a method for producing driving power in a paper or board mill, and equipment for producing driving power in a paper or board mill.
In this application, production of driving power means production of at least heat, steam, negative pressure, positive pressure, blow and electricity.
In a paper or board mill, heat energy, negative pressure, compressed air, blowing and electric energy are used in several places. The heat energy used may be in the form of e.g. hot air blast or in the form of steam.
Much heat energy and negative pressure is used in the paper or board mill's drying section in particular, and in state-of-the-art paper or board mills the drying section forms a bottleneck. With increasing paper or board machine speeds the drying section must be made longer in order to achieve a sufficient drying power. In addition, at high speeds various runnability components must be used, such as suction boxes and vacuum rolls, that is, so-called VAC rolls, in order to improve runnability. Such runnability components of different kinds are used both in the drying section and also e.g. in the formation section and in the press section.
In the drying section of state-of-the-art paper or board mills the paper or board is traditionally dried by drying cylinders using steam as their heat source. The steam required is usually produced in a separate steam production plant located in connection with the paper or board mill. The plant in question may be one producing only steam or one producing both steam and electricity. The paper or board mill's drying section applying cylinder drying is usually constructed so that every second roll is a heated drying cylinder while every second roll is a VAC roll. Hereby the web travels along a zigzag path from the drying cylinder to the VAC roll and from the VAC roll to the drying cylinder.
The negative pressure needed in the paper or board mill is usually produced by several separate vacuum pumps. These vacuum pumps are of large physical size and the driving motor of each vacuum pump is a high-powered, and thus also big, electric motor. The coefficient of efficiency of such vacuum pumps is very low, whereby it is very expensive to produce the negative pressure.
In a paper or board mill compressed air is used e.g. in the web transfers and in threading and also in various pieces of compressed air equipment. Compressed air is also used for producing negative pressure, e.g. in various blow-suction boxes. Negative pressure is produced in such places where it is possible using compressed air, because production of negative pressure by vacuum pumps is very expensive. The compressed air is produced by separate compressors, the driving motor of which may be an electric motor.
In the latest drying section solutions of state-of-the-art paper or board mills, drying is boosted by so-called impingement drying units. Metso Paper, Inc. markets such impingement drying units under its OptiDry trademark. The paper is taken on to the outer surface of the shell of the VAC roll having a relatively large diameter, where it will move through nearly a full revolution. In addition, in connection with the outer surface of the VAC roll at least one impingement unit is fitted, which is used to blow air at an approximate temperature of 350° C. against the web surface at an approximate speed of 90 m/s. The impingement unit includes a burner, a blowing fan and a lot of electric technology. The combustion gases of the impingement unit's burner are blown by a blowing fan towards the web surface. Some advantageous fuel such as natural gas may be used as the energy source for the impingement unit's burner. A vacuum pump is used to produce the negative pressure for the VAC roll of the impingement drying unit.
The following is a presentation of some state-of-the-art publications, which present paper or board mills applying impingement. By these references the concerned publications are included in the present application.
Metso Paper, Inc.'s FI patent 104100 presents an integrated paper machine using impingement in the drying of paper. The publication presents a pre-drying section, which is formed by a suction cylinder having a perforated shell and a diameter of 8–20 m and by a connected impingement device. Reference is also made in the publication to a planar pre-drying section equipped with impingement and to a pre-drying section of the Condebelt type.
Metso Paper, Inc.'s U.S. Pat. No. 6,101,735 presents several different drying sections of a paper machine applying impingement. The publication presents a planar impingement drying section and a drying section, where impingement units are fitted in connection with large-diameter drying cylinders.
Metso Paper, Inc.'s FI patent application 20002628 presents a drying section in a paper or board machine applying impingement. This uses a suction roll having a large diameter, preferably over 10 m, and impingement units fitted in connection with this.
Metso Paper, Inc.'s U.S. Pat. No. 5,306,395 for its part presents a tissue machine using a large-diameter suction roll and impingement. Here the press section of the tissue machine is replaced by a so-called TAD pre-drying section. The TAD pre-drying section is formed by a large-diameter suction roll, and an impingement unit is fitted in connection therewith. The impingement unit is used to blow hot air through the web traveling on the suction roll surface. From the TAD pre-drying section the web is moved on to the surface of a Yankee cylinder. In connection with the Yankee cylinder an impingement unit is also fitted, which is used to blow hot air against the web.
In state-of-the-art paper or board mills the following are needed in order to bring about the above-mentioned functions:                a steam production plant together with related systems for producing steam,        vacuum pumps to produce negative pressure, driving motors for the vacuum pumps, that is, electric motors, control systems and lubrication systems,        compressors to produce compressed air for peripheral equipment,        burners and blowing fans for the impingement units, electric drives for the blowing fans with controls, current sources etc.        
State-of-the-art paper or board mills thus need many separate systems in order to produce the above-mentioned functions. Separate systems are expensive, they make the system complicated and they contain a lot of details where failure may occur and which need maintenance. In addition, energy losses occur in all the above-mentioned systems, which results in a poorer total coefficient of efficiency and thus in poorer economy for the system.